Electricity
The purpose of The Tesla* Primer is to give added understanding to anyone who may have already acquired an electric vehicle (EV), those who may be contemplating acquiring one, or those who are merely questioning and want to be able to carry on a reasonable conversation on the topic. People are likely conversant when it comes to cars that burn gasoline or diesel. They would be familiar with fuel consumption i.e. miles per gallon (MPG) and have the skill to estimate driving range between tank fulls. Also, horsepower and torque; possibly transmissions, oil changes and annual emissions inspections. Electricity introduces a new vocabulary set and for EV management and monitoring an altered skill set. This primer will get you up to speed.
*While referring to TESLA™ in the title of this article, the body of information contained herein also will apply to the care and feeding of other manufacturer makes and models of EVs.
Electricity, sometimes referred to as power, electrons, current, or juice is a supplied force that does work for us. It is typically measured in Watts (W)*.
*There are 2 other terms to describe electricity: Volts and Amps. Think of Volts (V) as the pressure and Amps (A) as the capacity or speed that we move it. Watts (W) measures quantity. So if the flow of electricity is controlled by volts and amps then watts is the measured output.
The need for this term [watts] is in the necessity of describing — how much of something. You may already have a conception of watts. A 900 W hairdryer makes more heat than a 60 W light bulb. That is to say the 900 value represents more energy than 60 because more work happens.
These two consumers, the blow-dryer and light-bulb require 115 volts. A wall outlet in your house is a nominal 115-120 V. Further, if you have ever gone to the master panel to reset a [tripped] circuit breaker(CB) for an outlet you could read the label there that says the breaker is rated at 15 A. This voltage and amperage spec is typical throughout your residence. It is sufficient to provide power for several appliances, even simultaneously. Now we bring watts back into the picture and this top tip:
V × A = W i.e. Volts times Amps equals Watts
We may read from an appliance label some of or all of the 3 values but with the formula, if only 2 are known, then we can derive the 3rd. We know the standard 115 V and we know that the hair dryer labeled output is 900 W. Then what are the amps? Solve: 900 divided by 115 is 7.8 Answer: 7.8 amps.*
*Why care? Volts, Amps may surface when discussing charging and for sure when planning a charger installation in your garage.
Plug the 900 W hairdryer into the wall outlet it should operate as normal but to be sure let’s try the formula to evaluate. Calculate the maximum wattage available to item(s) plugged into the 115 V outlet which is limited to 15 amps by the CB. Solve: 115 x 15 = 1,725 Answer: 1,725 W*. If we exceed this wattage number the circuit goes dead because the CB tripped as was intended**. To keep home construction costs down there are typically a number of wall outlets sharing the circuit and all protected by this lone 15 A CB. Say a second hair dryer is plugged in and switched on. It will overload and exceed the design limitation. Why? Because now we are pulling 1800 W (2 X 900 W hairdryer) So, using the formula of which 2 values are known we can derive the 3rd value — how many amps (1800 / 115 = 15.6) The 15.6 A that we asked from the circuit exceeded the CB rating.
*Whenever you start multiplying anything the size of numbers can get out of hand. To keep them short and simple drop some zeros. So, in the above exercise One thousand seven hundred and twenty five watts is typically abbreviated by converting our number to kilowatts (kW)*. Therefore 1,725 W is expressed as 1.725 kW. We divided our watts by 1000 to get kilowatts. Rounding the decimal if necessary might even make it more manageable e.g. 1.7 kW. Hint: 1,000 watts = 1 kW
**not exactly… safety code is even more conservative and dictates that we not try and draw more than 80% of the rated value. So, for example if you had a 50 A CB you wouldn’t expect to draw more than 40 A from it. This 20% buffer margin is intended to keep you from running to the service panel for frequent resets or critically, the wires from getting too toasty warm and burning down the house!
Dueling hair dryers may be a rare happenstance but there are certainly larger energy consumers in our houses; the clothes dryer for instance. It uses a whopping 6,000 W. The 115 V wall socket just won’t cut the mustard. The solution is to raise the pressure and the flow i.e. more volts and amps. Your residence has 2,3 or more specially up-gauged (thicker wire) circuits that supply a nominal 240 V*. This will do the job for big loads — air conditioning, stove top / oven, clothes dryer and EV charging.
*Why aren’t all of the household circuits wired for 240? Answer: Cost.
Watts and Kilowatts are also expressed as a quantity over the course of time. The time element used would be hours (h). Example, if you were to switch on a table lamp with that 60 W bulb for an hour’s time the energy used would be 60 watt/hours expressed this way: 60 Wh. That light switched on for 2 hours would use 120 Wh.* The clothes dryer running for 1 hour would consume 6 kWh
1 kWh = 1,000 Wh
*Take a look at your utility bill. This is how it’s done. They bill you for each kW/h used. The meter outside conveniently reads in kW/h. So, if your billing rate is $0.14 per kWh and you used N kWh you can do the multiplication of N to arrive at your dollar total. Figure by adding up the consumers and the watts that they consume for the number of hours they consumed them and you get N kW/h that the meter recorded.Add taxes and fees, demand charges, surcharges (oh my!) for the grand total
More fun with appliances: They have an Underwriters Laboratory (UL) label visible somewhere on which you can see the energy requirements. I inverted the kitchen toaster to see the label. It read 115 V and 1600 W. (It can toast four slices at once) I happen to have a handy gadget that works as a portable meter when plugged between the appliance and the wall outlet. Just for kicks, I tried it with the toaster and then next on an electric teapot. The teapot had a cup of water hot and ready in 1 minute 34 seconds.
Teapot Result Readings: 121.3** V and 12.33 A
The gadget is smart and does the computation for you but let’s do the math using the formula. volts times amps equals watts
121.3 × 12.33 = 1,495.629
If we let the teapot boil away it would use the 1,496 W for as long as we let it go. But since my cup of water was hot in only 94 seconds at which point I cut power and then drank the cup’s contents, how much energy was used? Recall that 1,495 watts for 1 hour == 1,495 Wh. Well, there are 3,600 seconds in an hour so divide the 1,495 Wh by 3,600 and we get 0.4154525 Wseconds then multiply this by the 94 seconds. The total is 39.052535 Wh.
(1,495.629 ÷ 3,600) × 94 = 39.052535
39 Wh converted to kWh is 0.039 Just for grins, when you multiply this kWh result times a typical utility rate of say $0.14 per kWh then you get a cost of $0.01*
*They make up for it though when you run the clothes dryer (because, lots of watts!) The utility meter outside does the work of tracking a running sum total for you the utility company.
**Why did the meter gadget show [121.3] voltage available for the teapot other than the expected standard of 115? Because, 115 V is only a target. The utility company makes an effort but your value may vary depending on time of day, what you and your neighbors are pulling from the grid, and distance between your place and the utility’s transformer equipment located elsewhere in the hood. A range of 110 to 125 volts is not unusual. Similarly, 220 to 250 on the heavy-duty circuit is not uncommon either.